This game compares drainage area: the land area that funnels water into a river system.

• A bigger basin means more territory drains toward that river, not that the river is automatically longer or faster.
• Drainage area is a watershed statistic, so confluences and basin boundaries matter a lot.
• Tributaries can have enormous basins of their own, which is part of what makes river systems weirdly humbling.

Some rivers are less a channel than a continental funnel.